Mild cosmetic compositions must satisfy a number of criteria including cleansing power, foaming properties and mildness/low irritancy/good feel with respect to the skin, hair and the ocular mucosae. Skin is made up of several layers of cells which coat and protect the underlying tissue. The keratin and collagen fibrous proteins that form the skeleton of its structure. The outermost of these layers is referred to as the stratum corneum. Hair similarly has a protective outer coating enclosing the hair fibre which is called the cuticle. Anionic surfactants can penetrate the stratum corneum membrane and the cuticle and, by delipidization destroy membrane integrity and loss of barrier and water retention functions. This interference with skin and hair protective membranes can lead to a rough skin feel and eye irritation and may eventually permit the surfactant to trigger immune response creating irritation.
Ideal cosmetic cleansers should cleanse the skin or hair gently, without defatting and/or drying the hair and skin and without irritating the ocular mucosae or leaving skin taut after frequent use. Most lathering soaps, shower and bath products, shampoos and bars fail in this respect.
Certain synthetic surfactants are known to be mild. However, a major drawback of some mild synthetic surfactant systems when formulated for shampooing or personal cleansing is that they have what can be described as a "slippy" or "non-draggy" rinse feel which is not liked by some consumers. The use of certain surfactants such as potassium laurate, on the other hand, can yield a "draggy" rinse feel but at the expense of clinical skin mildness. These two facts make the selection of suitable surfactants in the rinse feel and mildness benefit formulation process a delicate balancing act.
Thus a need exists for personal cleansing compositions which deliver a "draggy" rinse feel while at the same time having excellent skin mildness, in addition to excellent product characteristics such as lather, cleansing, stability, thickening, rheology and in-use skin feel attributes.
Certain polyalphaolefin oils are known for use in personal cleansing compositions for the skin and hair. References to the use of such oils in personal cleansing formulations are to be found in WO 97/09031, U.S. Pat. No. 5,441,730, WO 94/27574, EP-A-0692244, WO 96/32092 and WO96/06596. Hydrophobically modified silicones oils are also known for use in personal cleansing compositions and are disclosed for example in JP 05-310540.
Surprisingly, it has now been found that personal cleansing compositions having a "draggy" rinse feel at the same time as having excellent mildness characteristics are provided by the combination of certain water-insoluble oils, such as certain polyalphaolefin oils or hydrophobically modified silicones oils, in combination with a mild, water-soluble surfactant system.
Whilst not wishing to be bound by theory, the "draggy" rinse feel is considered to be associated with an increase in wet skin friction. An important mechanism for action of such oils is considered to be their ability to deposit and change the surface energy of the skin, i.e. making the skin surface more hydrophobic. During rinsing, the water film is considered to be the lubricant for the skin, and as surface hydrophobicity increases so the water film is destabilised and the surface de-wetted. As a result the water film is at first thinned and then displaced, allowing some direct contact between the surfaces. Both changes increase friction and produce "draggy rinsing".